The processing of semiconductor wafers has become of great economic significance due to the large volume of integrated circuits being produced and the significant value associated with such circuits. Competitive pressures have driven dramatic changes in production. Foremost among these is the reduction in size of the various features of an integrated circuit which make up the transistors and other devices being formed on the integrated circuits. This reduction in feature size has been driven to achieve greater levels of integration, more sophisticated and complex circuits, and to reduce production costs by obtaining more integrated circuits on each wafer being produced.
Even though feature sizes used in integrated circuits have decreased dramatically, additional reductions are continuously being pursued. The temperature at which wafers are processed has a first order effect on the diffusion of dopants, deposition of materials or other thermal processes being performed. Thus it is important to have processing equipment which can achieve accurate temperature control to meet desired thermal processing specifications. As feature size decreases, the importance of accurate temperature control during processing increases to even a greater degree.
Traditionally, semiconductor thermal reactors have used Proportional-Integral-Derivative (PID) controllers to control temperature. Although such controller have the advantage of easier operation and maintenance, they are of limited accuracy in controlling temperatures. This limited accuracy imposes limitations on the achievable size and yield of integrated circuits.
The more typical PID controller parameters are experimentally tuned by adjusting the gain values or selected using a variety of tuning rules (e.g., Ziegler-Nichols). Such control methods give relatively less accurate control of the temperature of the thermal reactor with associated limitations on production yield and consistency of the resulting integrated circuits or other semiconductor items being produced.
More complex control schemes have been devised, but these more complex schemes frequently are so complex computationally that on-line operation is either not possible or not feasible. More complex control schemes have also in some cases not been used because of difficulties in achieving required control system tuning and maintenance at the production facility. This has been of somewhat greater concern with regard to controllers operated by engineers who do not have strong control system backgrounds. As a result is it often difficult for them to resolve all of the complexities imposed in adjusting the control system to the variations in their specific processor performance. This is exacerbated by variations in the same processor with time and changing conditions.
The temperature control problems encountered in thermal processing of semiconductor devices can be thought of in several different ways. One control problem involves matching the wafer temperature to the desired overall or average target or recipe temperature of the processor. The problem involves both achieving the desired recipe temperature and in achieving relatively consistent temperatures from one production run to another.
The desired overall or average recipe temperature of the processor can conveniently be thought of in terms of three different phases. The first phase is typically a ramp-up phase wherein the average operating temperature increases or ramps from a low level when processing is begun. The temperature ramp-up phase is thereafter typically followed by a period during which a desired maximum or other constant processing temperature is maintained. Such a constant temperature phase includes a stabilization period during which the changing temperature ramp ends and a constant or near constant temperature is achieved. Constant temperature phases may occur one or more times in a processing cycle. A further phase is the ramp-down phase wherein the average temperature of the processor in decreasing. Appreciate that various processes may include more than one of each of these three different phases.
Whether simple or more complex temperature plans or recipes are used, each phase may further be complicated by the introduction of one or more supplementary processing gases or vapor phase processing constituents which affect temperature and thermal response. Such supplementary processing gases are typically gases containing dopants, deposition materials or steam.
Another temperature control problem is to achieve relatively similar temperature exposures or histories during a processing cycle for each of the wafers or other semiconductor workpieces being processed within a batch. Temperature variations routinely occur with regard to wafers positioned near the ends of the array of wafers held within a processing furnace. There may also be other less predictable variations from wafer to wafer, such as along the array of wafers contained within the processing array.
A still further temperature control problem is associated with temperature variations across an individual wafer or other semiconductor workpiece being processed. This area of variability is exemplified by the geometry of most processing furnaces which have a grouping of multiple electrical heating elements formed in rings which surround the array of wafers being processed. Heat from the heating elements is being radiated through the processing vessel and variations can occur with regard to the heat gain experienced by the peripheral areas of the wafer as compared to the interior areas. Variations in the degree of radiant heat transfer and radiant shadowing which occur from wafer to wafer further exacerbates this problem.
Another noteworthy consideration is the manufacturing concern to minimize the processing time used to effect a particular process or group of processes being carried out with the thermal processor. Minimizing the processing time will typically increase the ramp-up phase temperature change rate. Conversely, time concerns will also increase the ramp-down phase temperature change rate. Increased rates of temperature change cause greater difficulties in maintaining recipe temperatures during the processes of transitioning between ramp-up and stabilization phases, and between stable temperatures and relatively rapid temperature ramp-down phases.
Given these complexities and somewhat countervailing considerations, there is great difficulty in achieving improved control systems which are both practical and workable for improved thermal processing of semiconductor wafers.